1. Field of the Invention
The present invention relates to an automated focus point control device, and more specifically relates to an automated focus point control device for use with microfilm readers and the like.
2. Description of the Related Art
In conventional microfilm readers and the like, automated focus point control devices (autofocus mechanisms) are known which automatically focus the film image when forming said microfilm image on a screen or the like. Such an automated focus point control device comprises photoreceptor elements such as CCD line sensors or the like which detect the film image passing through a lens, and a control device for controlling the lens position and the like. When a film image is focussed in the CCD line sensor, the difference between the light portions and the dark portions of the film image (image contrast) is at its greatest. The control device is used to control the position of the lens so as to align the focus of the film image projection detected by the CCD line sensor.
When the contrast of the film image area detected by the CCD line sensor is slight, e.g., when the film image area lacks distinct edges or the like, it is difficult to accurately judge whether or not the image is in focus. In such a case, the aforesaid automated focus point control device searches for the area of highest contrast in the film image and it is this area which is detected by the CCD line sensor so as to accurately control the autofocus function.
However, the automated focus point control device of the aforesaid type must search for a film image area having a contrast of a predetermined value or greater. Consequently, there is an inherent disadvantage inasmuch as substantial time may be required to accomplish said autofocus control.
Automated focus point control devices have also been provided wherein the autofocus is controlled by detecting an edge portion of a blip mark used for detecting an image recorded outside the frame of the film image. The aforesaid blip mark typically is a rectangular graphic image recorded in the white area of the film.
The position and size of the aforesaid blip mark differs in accordance with the type of film and the like. Therefore, the position of the blip mark must be detected by the CCD line sensor or the like prior to detecting an edge portion of said blip mark. Accordingly, the automated focus point control device of the aforesaid type also has certain inherent disadvantages inasmuch as a long time may be required to adjust the focus.
Two types of methods are used in the automated focus point control device to detect the area of highest contrast within the film image. These two types of methods are described hereinafter.
A first method moves the projection lens between a position nearest the object side to a position nearest the image side and sequentially calculates the difference in contrast between the contrast of the former position and the contrast of the subsequent position, such that the point at which the value of said difference changes from a positive value to zero or a negative value is designated the focus position.
A second method detects the contrast throughout the entire range from a position nearest the object side to a position nearest the image side, and designates the position of maximum contrast therebetween as the focus position.
However, the aforesaid second method must move once the optical elements of the image forming system from a position nearest the object side to a position nearest the image side to detect the contrast throughout the entire range. The automated focus point adjustment therefore requires a long length of time because the optical elements of the image forming system must be again moved to the focus position after the focus position is determined.
The aforesaid first method also requires a long length of time for automated focus point adjustment when the focus position of the optical elements of the image forming system is separated from the start position of the aforesaid movement of the image forming system.